With the recent rapid prevalence of the Internet, there has been a demand for a further increase in the capacity of communication traffic. To deal with this, efforts have been made, in the field of optical communication systems, to increase the number of available channels by improving the transmission rate per channel and by utilizing a wavelength division multiplexing (WDM) scheme.
The WDM is a scheme to simultaneously transmit a large number of optical signals with different carrier wavelengths (channels). The WDM allows transmissions via a single optical fiber, thus enabling communication capacity to be increased consistently with the number of channels.
For example, when 100 channels are transmitted via one common optical fiber based on modulation at 10 Gigabits/sec per channel, the communication capacity can be increased to 1 terabits/sec.
The recent medium- and long-distance optical communication commonly uses a C band (1,530 to 1,570 nanometers) that can be amplified by an optical fiber amplifier (EDFA, Erbium Doped Fiber Amplifier). Furthermore, to allow a further increase in communication capacity, an L band (1570 to 1610 nanometers) may be used.
In general, the WDM system requires different laser devices for respective wavelengths. Thus, the manufacturer and user of the WDM system need to prepare laser devices corresponding to the wavelengths of standard channels. For example, 100 channels require the respective, 100 types of laser devices, resulting in an increase in the need for inventory control and an increase in inventory costs.
Thus, in WDM systems, particularly medium- and long-distance communication systems, there has been a demand for practical application of a wavelength tunable laser device allowing all the wavelengths of the C band (or L band) to be covered by the single laser device. When the entire C band (or L band) can be covered by the single laser device, the manufacturer and user have only to prepare the single laser device. Thus, the need for inventory control and inventory costs can be sharply reduced.
In order to construct a high-capacity, high-performance, and reliable optical communication network, it is essential to provide a technique for allowing a light emitting device to freely change and to control the wavelength. A wavelength tunable laser device built into the communication system is a very important key device for controlling of the wavelength.
Many methods for implementing a wavelength tunable laser device that meets these demands have been proposed.
In this connection, Japanese Patent Laid-Open No. 2003-023208 (hereinafter referred to as Patent Document 1) describes a structure in which a large number of distribution feed-back semiconductor laser (DFB laser) elements with different laser oscillation wavelengths are arranged in parallel. In the structure, the semiconductor laser elements are switched for rough adjustment and a variation in refractive index caused by temperature is utilized for fine adjustment.
However, the structure requires an optical coupler to collectively couple output ports of the large number of laser elements to an optical fiber. Thus, an increase in the number of laser elements arranged in parallel correspondingly increases the loss of the coupler. Consequently, the variable range of the wavelength and an optical output are in a trade-off relationship.
On the other hand, much effort has been made to research and develop external resonator-type wavelength tunable laser devices expected to avoid the above-described trade-off relationship to meet the demand for wavelength control.
The external resonator-type wavelength tunable laser device uses a semiconductor optical amplifier (SOA) corresponding to a semiconductor gain region and an external reflection mirror to form a resonator. A reflection structure enabling the wavelength to be varied (wavelength tunable reflector) is inserted into the resonator to allow the wavelength to be selected. The external resonator-type wavelength tunable laser device relatively easily provides a wavelength tunable range that covers the entire C band.
Most of the basic characteristics of wavelength tunable laser devices of this type are determined by the wavelength tunable reflector. Thus, various wavelength tunable structures with excellent characteristics have been developed. For example, Japanese Patent Laid-Open No. 04-69987 (hereinafter referred to as Patent Document 2) and Japanese Patent Laid-Open No. 2000-261086 (hereinafter referred to as Patent Document 3) disclose reflection structures using an acoustic engineering filter, a dielectric filter, an etalon, or the like.
A variety of external resonator-type wavelength tunable laser devices use such a wavelength tunable structure. In particular, a configuration that has, in addition to a gain medium, a periodic channel selection filter, a wavelength tunable filter, and a reflection mirror as disclosed in Patent Document 3 is effective for providing a high-performance light source.
For example, an etalon, which offers periodic frequency characteristics, is used as a periodic channel selection filter. Furthermore, an acoustic engineering filter is used as a wavelength tunable filter. An electrically controlled wavelength tunable reflector is used as a wavelength tunable reflector.
An electrically drivable, planar wavelength tunable reflector is effective for providing a small, inexpensive, and reliable external resonator-type wavelength tunable laser device. Such a structure is disclosed in, for example, Non-Patent Document 1 described below.
[Non-Patent Document 1] J. De. Merlier et. al., “FullC-Band External Cavity Wavelength Tunable Laser Using a Liquid-Crystal-Based Tunable Mirror”, IEEE PHOTONICS TECHNOLOGY LETTERS, March, 2005, Volume 17, No. 3, pp. 681-683 (FIG. 1(a)).
On the other hand, a fiber Bragg grating laser is known as an external resonator-type laser device including a combination of a semiconductor optical amplifier and a wavelength-fixed reflection mirror. The principle of the fiber Bragg grating laser is described in, for example, Japanese Patent Laid-Open No. 11-214799 (hereinafter referred to as Patent Document 4) in detail.
Even if the reflection peak wavelength of the fiber Bragg grating is different from the gain peak wavelength of the semiconductor optical amplifier, provided that the two peak wavelengths are close to each other, the laser device can be oscillated at the reflection peak wavelength of the fiber Bragg grating. On the other hand, when the two peak wavelengths are significantly different, the laser device oscillates at the gain peak wavelength of the semiconductor optical amplifier.
The range of the difference between the reflection peak wavelength of the fiber Bragg grating and the gain peak wavelength of the semiconductor optical amplifier needs to be properly set in order to allow the laser device to oscillate at the reflection peak wavelength. The range to be achieved is called an available wavelength range.
However, it has been found that when a planar wavelength tunable reflector is used to construct an external resonator-type wavelength tunable laser device in an attempt to obtain wavelength tunable characteristics, the laser device may disadvantageously oscillate at a wavelength that is different from the reflection spectral peak of the wavelength tunable reflector used.
The cause of this phenomenon has been examined and found to be that the planar wavelength tunable reflector involves a considerable level of reflection (hereinafter referred to as residual reflection) at wavelengths that are different from the reflection spectral peak.
The reflection spectral peak wavelength of the wavelength tunable reflector is defined as The reflectance of the wavelength tunable reflector obtained at wavelength λ1 is defined as R1(%). The reflectance of the wavelength tunable reflector obtained at wavelength λ2 that is different from the wavelength λ1 is defined as R2(%). Reflectance difference ΔR (dB) is defined by ΔR=10·log(R1/R2). Then, some devices only have reflectance difference ΔR of about 6 dB to 8 dB.
Conventionally, a semiconductor optical amplifier corresponding to a semiconductor gain region combined with an external resonator-type laser (for example, a fiber Bragg grating laser) normally involves a relatively large number of, about 6 to 10 layers of quantum wells and a large element length of at least 1,000 μm because the external resonator generally suffers a great loss.
For example, if a fiber Bragg grating laser is constructed by combining a semiconductor optical amplifier including a multiple quantum well (MQW) with eight active layers and having an element length of 1,000 μm and a planar reflection structure with a reflectance difference of 6 dB to 8 dB, the wavelength range will be between about 30 nm and 35 nm.
The wavelength range of a wavelength laser such as a fiber Bragg grating laser is within the available wavelength range. The wavelength range of such a single-wavelength laser thus corresponds to the working range of gain peak wavelength λp of the semiconductor optical amplifier within which the laser device can be oscillated at intended wavelength λB specified for the fiber Bragg grating. That is, the laser device may be produced such that gain peak wavelength λp is within the available wavelength range which is predetermined. The range of 30 nm to 35 nm is a sufficiently large tolerance and can be easily achieved.
However, this is disadvantageous to the wavelength tunable laser device. The disadvantage will be described below with reference to FIGS. 1 to 3.
FIG. 1 is a graph illustrating an available wavelength range in a simplified manner. As shown in FIG. 1, regardless of gain peak wavelength λp of the semiconductor optical amplifier, which is shown on the axis of abscissa, the laser device oscillates, within a certain range of gain peak wavelength λp (available wavelength range), at optical wavelength λB selected by a wavelength selecting reflection structure (fiber Bragg grating).
This is shown in FIG. 2 in a more simplified manner so as to be easily understood in the following description.
FIG. 2 indicates that if the gain peak wavelength of the semiconductor optical amplifier is within the range shown by an arrow as an available wavelength range, the laser device can oscillate at optical wavelength λB shown by an arrow. FIG. 3 illustrates the possible range of the gain peak wavelength of the semiconductor optical amplifier contained in the wavelength tunable laser device; the range is based on FIG. 2.
The wavelength tunable laser device has an intended wavelength tunable range such that the device needs to be able to oscillate at all selected wavelengths within the range. That is, as shown in FIG. 3, available wavelength ranges shown in FIG. 3 are set based on minimum wavelength λ5 and maximum wavelength λ6 of the desired wavelength tunable range. The overlapping part between the available wavelength ranges each including minimum wavelength λ5 or maximum wavelength λ6 is set to be the range of gain peak wavelength λp. Then; the wavelength tunable laser device can be operated by a single semiconductor optical amplifier.
As is apparent from FIG. 3, a narrower available wavelength range and a wider wavelength tunable range (a larger difference between wavelength λ5 and wavelength λ6) contribute to reducing the possible range of gain peak wavelength λp, which is difficult to achieve.
The wavelength tunable laser device taken above as an example and configured as follows offers only an available wavelength range of 30 nm to 35 nm, as described above: the laser device includes a built-in semiconductor optical amplifier comprising eight layers of MQWs and having an element length of 1,000 μm and a built-in planar wavelength tunable reflector with a reflectance difference ΔR of 6 dB to 8 dB.
Thus, the wavelength tunable laser device with a wavelength tunable range of 30 nm to 35 nm or more fails to offer the possible range of gain peak wavelength λp of the semiconductor optical amplifier. That is, the desired wavelength tunable range of the wavelength tunable laser device cannot be achieved.
The above-described condition is a new limit on the wavelength tunable laser device. It is thus very difficult to provide a wavelength tunable laser device with the desired wavelength tunable range based on the control only of gain peak wavelength λp of the semiconductor optical amplifier.
This problem is particularly significant when the wavelength tunable range of the wavelength tunable laser device is set to be equal to or wider than the range of about 30 nm to 35 nm, generally corresponding to a full band operation in one communication wavelength band.
As described above, an external resonator-type wavelength tunable laser device using a wavelength tunable reflector involving residual reflection disadvantageously oscillates at a wavelength that is not similar to the one selected by the wavelength tunable reflector.
To solve this problem, the residual reflection in the wavelength tunable reflector may be reduced. However, in the planar reflection structure, setting ΔR to at least 20 dB is very difficult. With possible mirror production process techniques, ΔR is often set to less than 16 dB.